This invention relates to a process for the purification of a solid material. More particularly, this process relates to a process for purifying a solid material such as a metal, a metalloid, or a metal compound which has been heated to a temperature approaching the melting point of the material to be purified while contacting the heated material with a purifying agent which is substantially non-reactive with the solid material.
2. Description of the Prior Art
There is an increasing demand for high purity materials such as silicon, titanium, boron, gallium arsenide, silicon carbide, etc. for diverse applications such as solar cells, rocket fuel, high purity alloys, semiconductors, and nuclear fuel applications.
For example, an increasing demand for silicon of sufficiently high purity to be suitable for use in the semiconductor and solar cell industries has lead to investigation of many processes to achieve such purity levels. Such processes typically involve some sort of treatment of molten silicon. Purification of a material such as silicon in a molten state is, however, not new. For example, Allen U.S. Pat. No. 1,037,713 describes the purification of silicon by treating molten silicon with metals, such as alkali metals and alkaline earth metals including magnesium.
Brockbank U.S. Pat. No. 1,180,968 describes melting silicon under a slag of natural or artificial silica to eliminate impurities while Pacz U.S. Pat. No. 1,518,872 describes silicon as a valuable byproduct of a reaction between aluminum powder and a metallic fluorosilicate, such as magnesium fluorosilicate.
Pruvot et al U.S. Pat. No. 3,034,886 describes the purification of silicon or ferrosilicons by the injection of silicon fluoride gas into the liquid bath to react with aluminum and calcium impurities to form aluminum and calcium fluorides.
The use of molten metal fluorides for purification of silicon at a temperature of 1000.degree.-1600.degree. C. has been proposed by Coursier et al U.S. Pat. No. 3,148,131. The patentees, however, propose the use of metal fluorides which, in the main, either represent costly materials or materials known to react with silicon to form silicon fluoride and inject impurities in the silicon that are detrimental to its electronic properties.
Boulos U.S. Pat. No. 4,379,777 teaches passing powdered silicon through a plasma which apparently causes migration of the impurities to the surface of the molten silicon particles. After quenching, the particles are acid-leached to remove the surface impurities.
Kapur et al U.S. Pat. No. 4,388,286 combines vacuum refining of silicon with mixing the silicon with an effective fluxing agent, such as a fluoride of an alkali metal or an alkaline earth metal, to form a molten silicon phase and a slag phase.
One of us has also authored or coauthored papers which refer to the purification of molten silicon in contact with NaF in "Silicon Sheet for Solar Cells", by A. Sanjurjo published in the Journal of the Electrochemical Society, Volume 128, pp. 2244-2247 (1981) and "Fluxing Action of NaF on Oxidized Silicon", by L. Nanis, A. Sanjurjo, and S. Westphal published in Metallurgical Transactions B, Volume 12B, pp. 535-573 of the American Society for Metals and the Metallurgical Society of AIME (1981).
Not all prior silicon purification processes, however, involve the melting of silicon. Ingle U.S. Pat. No. 4,172,883 discloses a process for purifying metallurgical grade silicon by heating it to 800.degree. to 1350.degree. C. and contacting it with silicon fluoride gas which is said to react with the impurities causing them to deposit out. The aforementioned Coursier et al patent also speaks of purification temperatures below the melting point of silicon.
It is also known to purify materials such as silicon by acid-leaching of the material in powder form as well as by unidirectional solidification of the material. In the case of silicon, some of these processes may be less expensive than the conventional method for obtaining high purity silicon from chlorosilane reduced - pyrolyzed in H.sub.2 to produce pur polycrystalline silicon which can cost as much as 70 times the metallurgical grade silicon starting material. However, most of the other methods proposed either involve high costs or are of limited value in producing a very high purity silicon, such as needed for solar applications, i.e., a purity of 99.999 to 99.9999%.
In the unidirectional solidification method, such as zone melting purification or Czochralski crystal growth, the material is melted and then slowly cooled down in such a way that the heat loss and the solidification occur mostly in one direction. The chemical potential of an impurity in the solid material (.mu..sub.1) is higher than in the liquid material. As a consequence, the impurity will migrate toward the area of minimum chemical potential thus establishing a segregation between the solid and the molten phase. This segregation, in turn, results in the purification of the phase in which the impurity has the more positive (higher) chemical potential.
In the slagging method, a molten material to be purified (such as iron) is put in contact with another molten material (such as CaSiO.sub.3) called "slag". The slag wets the material to be purified, but is substantially non-reactive to this material. The chemical potential of an impurity in the material to be purified is typically higher than the corresponding chemical potential of the same impurity, or its corresponding ion, in the slag. As a consequence, the impurity will migrate from the molten material to be purified to the slag, thus resulting in purification of the material, e.g., the iron. The degree of purification can be estimated from the difference in the chemical potentials.